Two bombs have exploded in a busy market in the Kenyan capital as hundreds of British tourists prepare to return to the UK amid warnings of a "high threat" of terrorism.

At least 13 people were killed and more than 70 others wounded in the blasts on a minibus and in Nairobi's Gikomba Market.

Kenya's National Disaster Operation Centre said the explosions, condemned by Foreign Secretary William Hague as "appalling acts of violence", were caused by homemade bombs and that one person had been arrested.

Nairobi police said they are holding one suspect in connection with the two explosions.

The scene of one of two explosions in Nairobi

The Foreign and Commonwealth Office (FCO) has warned Britons of a "high threat" of terrorism.

Similar advice has been issued by the governments of France, Australia and the US, prompting travel company Thomson to begin evacuating its 400 customers from the country.

Video:First Kenya Flights Back In UK

Holidaymakers are being advised to leave unless there is "an essential reason to remain".

A Sky News source said the threat was not specific to British people and no precise plot had been uncovered.

"The advice to consider leaving is due to a general worsening of the security situation over time," the source said.

Video:Blasts As Brits Told To Leave Kenya

Another source claimed there was "an increased threat to Western nationals since Westgate" - a reference to the attack on the Westgate shopping centre in Nairobi last September by the Somali militant group al Shabaab in which at least 67 people died.

The FCO website has a map of Kenya marking the areas of concern. These include anywhere within 60km of the Kenya-Somali border, the Eastleigh area of Nairobi and Mombasa Island.

The advisory does not cover tourist areas of Diani or Moi international airport which serves the city of Mombasa.

Video:Improvised Devices Rip Market Apart

Explaining the threat, the advice reads: "There is a high threat from terrorism, including kidnapping. The main threat comes from extremists linked to al Shabaab, a militant group that has carried out attacks in Kenya in response to Kenya's military intervention in Somalia.

"There has been a spate of small-scale grenade, bomb and armed attacks in Nairobi (especially the area of Eastleigh), Mombasa, and North Eastern Province."

The US cited hotels, nightclubs and shopping centres in Mombasa as possible targets.

One flight carrying returning holidaymakers arrived in Gatwick airport on Friday morning and another was due in late on Friday. They had been scheduled to fly on May 20 and 27.

Kenyan authorities have pledged to beef up security but insisted visitors were safe.